4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific
nitrosamine, induces lung
adenomas in A/J mice, following a single intraperitoneal (i.p.) injection. However, inhalation of tobacco
smoke has not induced or promoted
tumors in these mice. NNK-induced lung
tumorigenesis is thought to involve
O6-methylguanine (O6MeG) formation, leading to GC-->AT transitional mispairing and an activation of the K-ras proto-oncogene in the A/J mouse. NNK can be metabolized by several different
cytochromes P450, resulting in a number of metabolites. Formation of the promutagenic
DNA adduct O6MeG is believed to require metabolic activation of NNK by
cytochrome P450-mediated alpha-hydroxylation of the methylene group adjacent to the N-nitroso
nitrogen to yield the unstable intermediate, methanediazohydroxide.
Nicotine,
cotinine (the major metabolite of
nicotine), and aqueous
cigarette tar extract (ACTE) have all been shown to effectively inhibit metabolic activation of NNK to its mutagenic form, most likely due to competitive inhibition of the
cytochrome P450 enzymes involved in alpha-hydroxylation of NNK. The objective of the current study was to monitor the effects of
cotinine and cigarette
smoke (CS) on the formation of O6MeG in target tissues of mice during the acute phase of NNK treatment. To test the effect of
cotinine, mature female A/J mice received a single
intraperitoneal injection of NNK (0, 2.5, 5, 7.5, or 10 mumole/mouse) with
cotinine administered at a total dose of 50 mumole/mouse in 3 separate i.p.
injections, administered 30 min before, immediately after, and 30 min after NNK treatment. To test the effect of whole
smoke exposure on NNK-related O6MeG formation, mice were exposed to
smoke generated from Kentucky 1R4F reference cigarettes at 0, 0.4, 0.6, or 0.8 mg wet total
particulate matter/liter (WTPM/L) for 2 h, with a single i.p. injection of NNK (0, 3.75, or 7.5 mumole/mouse) midway through the exposure. Cigarette
smoke alone failed to yield detectable levels of O6MeG. The number of O6MeG adducts following i.p. injection of NNK was significantly (p < 0.05) reduced in both lung and liver by
cotinine and by cigarette
smoke exposure. Our results demonstrate that NNK-induced O6MeG
DNA adducts in A/J mice are significantly reduced when NNK is administered together with either
cotinine, the major metabolite of
nicotine, or the parental
complex mixture, cigarette
smoke.